This specification relates to criss-cross writing of microlithographic patterns with improved isotropy of writing characteristics from a writing mechanism having non-isotropic properties resulting from different degrees of coherence in a sweep direction and a cross-sweep direction.
Increasing requirements for critical dimension precision have led to a variety of writing strategies that reduce both occasional irregularity in microlithographic patterns and periodic errors, such as snap errors and biases. One of the biases can be explained with reference to FIG. 4, which has axes labeled X and Y. This illustrates using an acousto optic deflector, rotating mirror, or other deflector to sweep interlaced writing beams parallel to the Y axis, as relative movement is created between the workpiece and writing mechanism along the X axis.
In many writing architectures there is a mismatch between the image properties in different directions. In particular this is true for scanned laser beams being modulated by acoustooptic modulators and for systems based on scanned image of a one-dimensional diffractive SLM. The reasons are the same for other types of systems. There is a first direction where the image is built sequentially by the sideways addition of linear sub-images, so that two adjacent pixels in this direction are formed at different times and do not interfere optically. In another direction, typically perpendicular to the first direction, adjacent pixels are formed more or less simultaneously. This is obviously the case with a one-dimensional SLM illuminated to have some coherence between adjacent pixels. But even in the case of a scanning laser beam modulated by an acoustooptic modulator there are transition moments from one pixel to the next one in the modulator where parts of two pixels simultaneously modulate the beam and thereby cause some amount of interference. The phenomenon is caused by the finite velocity of sound in the acoustooptic modulator and may be present in various amounts in systems using acoustooptic, holographic, rotating, oscillating, or mechanical scanning of the beams over the surface of the workpiece. The interference between adjacent pixels is normally, and if properly designed, beneficial to the image quality. If the optical system has the same NA in both directions, lines with the edges defined by pixels which interfere (i.e. lines perpendicular to the second direction) have better resolution, or if the NA in this direction is adjusted for the same resolution these lines have significantly better depth of focus. There is also a difference in iso-focal dose, i.e. the dose which gives best practical depth of focus. There is a desire to make both directions print equally good, e.g. to have isotropic iso-focal dose behavior. Since the direction with the smallest depth of focus determines the practically useful focus latitude of the system, isotropic properties which are the average between the properties in the best and worst directions give an overall more capable system.
What has been described above is non-isotropy created by a different amount of coherence in the lengthwise/beam-scanning direction versus the cross-wise direction. The difference in coherence has profound influence on the image properties, e.g. affecting the iso-focal dose. Other systems may have other sources of non-isotropy and the technology disclosed may also be useful for reducing them
One effort to reduce errors is PCT application WO 2010/131239. In FIG. 11A, that reference teaches using a single direction of sweep, labeled 1010, and creating rectangular beamlets in an exemplary 2 by 3 array, with beamlets in one row being rotated +22.5 degrees and beamlets in the other row rotated −22.5 degrees, for a difference between the major axes of the two rows of rectangular beamlets of 45 degrees. The beamlets are generated from an SLM or DMD by optics that split the modulator output after modulation and rearrange the relative positions and orientations of the beamlets before they reach the workpiece, as illustrated in FIGS. 2 and 11A. In FIG. 2, the beamlets that the optics rearranges are labeled 181-184. In FIG. 11A, the rectangular beamlets that are rearranged and rotated are labeled 1000-1005 with 1000-1002 rotated +22.5 degrees and 1003-1005 rotated −22.5 degrees. The reference does not mention the problem of writing performance is bias in favor of accuracy along the so-called scan 1010 direction, with performance along the cross-scan 1011 direction being worse. To the contrary the reference teaches the use of a two-dimensional spatial modulator (DMD) which gives an image which is nearly isotropic to start with. The only non-isotropy in the partial images written by the fields in the reference comes from smearing of the pixels due to the movement, but both rotated images are smeared in the same direction. Therefore the anisotropy of the image is the same in the combined image as in the each partial image.
The so-called 1.5D SLM disclosed in a co-pending application by the same applicant is another way to reduce the non-isotropy of the image properties of a one-dimensional SLM scanned more or less sideways. The technology disclosed can be used together with the 1.5D SLM for a more complete elimination of non-isotropy.
An opportunity arises to improve pattern accuracy and reduce or eliminate bias and non-isotropy between scanning and cross-scanning directions in a microlithographic pattern. Better, more accurate systems may result.